Looks like you got a good finish with the sparex
I've been using the hammerite rust converter stuff - its white and then reacts with the rust, turning blue and finally finishes up black. It seems really good - even if you leave it with no top coat it'll still seem to last for going on a year. So I've been having a roll around underneath finding the corners and crevices.
I've also used Owatrol oil - in some places as a neat paint over - though it's easiest to use as a thinners in paint. Its very satisfying work.